Examples of a machine part having a contact surface which is mainly brought into rolling contact (that is, is brought into only rolling contact or is brought into rolling contact with sliding) include a roller for a cam follower, a bearing wheel, and a rolling element of a rolling bearing. Examples of the rolling element include a rolling element of a gear inner bearing which is incorporated into a rotation axis of a machine and is brought into direct contact with the rotation axis.
On the other hand, examples of a machine part having a contact surface which is brought into only sliding contact include a thrust washer which is brought into sliding contact while being pressed against an end surface of a roller.
Examples of the above-mentioned cam follower include one employed for a valve gear of the engine of an automobile, etc. The valve gear is constituted by a camshaft rotated while a crankshaft of the engine is rotated, and a mechanism for converting the rotation of the camshaft into vertical motion and transmitting the vertical motion to a poppet valve. Examples of the valve gear include ones of types such as a rocker arm type, a swing arm type, and a direct type depending on the difference in the transmitting mechanism.
In the valve gear, the rotation of the camshaft has been conventionally converted into vertical motion by a cam of the camshaft and a cam follower which is brought into sliding contact with an outer peripheral surface of the cam. In order to reduce the rotation load of the camshaft, however, valve gears of a construction in which a roller (a roller for a cam follower) is rotatably mounted on the cam follower, and an outer ring of the roller for a cam follower is brought into rolling contact with the outer peripheral surface of the cam of the camshaft, have increased in number in recent years.
In an OHC type engine or a DOHC type engine, for example, however, a camshaft is positioned above an engine casing, whereby lubricating oil is liable to be insufficiently supplied, and an outer peripheral surface of a cam is not cleanly surface-finished and is very rough, unlike a raceway surface of the bearing wheel and an outer peripheral surface of the rolling element of the rolling bearing. Therefore, a sufficient lubricating oil film is not formed on an outer peripheral surface of a roller for a cam follower.
Therefore, damage called peeling is produced on a rolling contact surface of the roller for a cam follower with the cam, to significantly reduce the life of an outer ring of the roller for a cam follower.
A technique for forming very small recesses on a sliding contact surface, using the recesses as an oil sump, and forming an oil film on the contact surface by the supply of oil from the oil sump has been proposed (see Japanese Patent Laid-Open No. 172608/1991, for example). It is considered that the technique is diverted into a machine part such as a roller for a cam follower.
However, there may, in some cases, be a case where sufficient durability is not obtained depending on how the very small recesses are formed.